1. Field of the Invention
The present invention relates to an electrocoating system for applying an electrodeposited coating layer to an object under sequentially applied different voltages, and more particularly to such an electrocoating system having means for preventing a coating from being deposited on electrodes themselves.
2. Prior Art
Electrocoating processes employing a cationic coating material, particularly for electrocoating automotive bodies, have heretofore applied a high voltage during electrocoating cycles to produce a coating of large thickness in order to make the coating resistant to rust.
The conventional electrocoating process in which a high voltage is applied to an object such as an automotive body is however disadvantageous in that the electrodeposited coating suffers various surface irregularities such as blisters, pimples, pinholes, and rivels. These coating problems are apt to occur on a steel plate composed of multiple constituents, especially on a surface-treated steel plate used for increased rust prevention.
One electrocoating system for preventing the formation of such irregular coating surfaces is disclosed in Japanese Laid-Open Patent Publication No. 58-93894, for example. According to the disclosed electrocoating system, a lower voltage is applied during an initial stage of coating formation on an object, and thereafter a higher voltage is applied during a subsequent coating stage.
The prior electrocoating system is shown in FIG. 4 of the accompanying drawings. The system includes a first-stage electrode 102 and second-stage electrodes 103a, 103b disposed in an electrocoating bath 101 and arranged successively from inlet to outlet sides of the bath 101. A voltage V1 applied to the first-stage electrode 102 is selected to be lower than a voltage V2 applied to the second-stage electrodes 103a, 103b.
The voltages V1, V2 are produced by rectifying voltages from a three-phase AC power supply 104 with rectifying circuits 105a, 105b comprising thyristors or the like. The different voltages V1, V2 are generated by controlling the firing phase of the thyristors.
The rectifying circuits 105a, 105b have negative terminals connected respectively to bus bars 106a, 106b. An object 107 to be coated such as an automotive body is held in slidable contact with the bus bars 106a, 106b through a current collector 108. The bus bars 106a, 106b can be electrically connected to each other by means of a switch 109.
The bath 101 is filled with a coating material 110 and grounded. The bus bar 106b which applies the higher voltage is also connected to ground. The object 107 is moved in the bath 101 in the direction indicated by the arrow A.
As shown in FIG. 5, there is a potential difference .DELTA.V1 between the voltage V1 applied to the first-stage electrode 102 and the voltage V2 applied to the second-stage electrodes 103a, 103b (V1&lt;V2). Therefore, an electric current flows from the second-stage electrodes 103a, 103b toward the first-stage electrode 102, which itself is gradually electrocoated.
When the first-stage electrode 102 is electrocoated, it can no longer serve as an electrode since the electrodeposited coating thereon is an electrical insulator. The coating material is then electrodeposited on the object 107 only by the second-stage electrodes 103a, 103b. The electric current which flows from only the second-stage electrodes 103a, 103b is so small that the thickness of the electrodeposited coating may be small or suffer surface irregularities as described above.
FIG. 5 shows potentials of the electrodes when the switch 109 is open, or rendered nonconductive. The potential difference .DELTA.VI between the first-stage electrode 102 and the second-stage electrodes 103a, 103b is equal to the difference between the voltage V2 applied to the second-stage electrodes 103a, 103b and the voltage V1 applied to the first-stage electrode 102. The electric current based on the voltage difference flows into the electrode plate 102 to electrodeposit the coating material thereon.
When the object 107 moves from the bus bar 106a to the bus bar 106b, the switch 109 is turned on or closed to keep the bus bars 106a, 106b at the same potential. If there were potential difference between the bus bars 106a, 106b at this time, when the bus bars 106a, 106b are electrically connected by the current collector 108, a spark discharge would be produced, damaging the current collector 108 and the bus bars 106a, 106b. When the switch 109 is turned on, there is developed a potential difference .DELTA.V2 (see FIG. 6) between the first-stage electrode 102 and the second-stage electrodes 103a, 103b, the potential difference .DELTA.V2 being the difference between the voltages applied to the electrodes 102 and 103a, 103b. The electrode 102 is thus electrocoated with more coating material under the increased potential difference .DELTA.V2.
In order to eliminate the above drawback, the applicant has proposed an electrocoating system as shown in FIG. 7 in which a diode 111 is forward-connected between first-stage electrodes 102a 102b and the positive terminal of a first-stage power supply 105A to eliminate a current loop from a second-stage power supply 105B to second-stage electrodes 103a, 103b to a coating material 110 to the first-stage electrodes 102a, 102b to the first-stage power supply V1 (see Japanese Laid-Open Patent Publication No. 62-156300).
The diode 111 prevents any electric current from flowing from the higher-voltage power supply V2 to the lower-voltage power supply V1, so that the coating material which would otherwise be deposited on the first-stage electrodes 102a, 102b is largely reduced.
With the plural electrodes 102a, 102b employed as shown in FIG. 7, however, an electric current flows from the second-stage electrodes 103a, 103b to the first-stage electrode 102b to the first-stage electrode 102a to a bus bar 106a, thus electrodepositing the coating material on the first-stage electrode 102a or 102b to form an insulative coating thereon.
This problem will be described in detail with reference to the equivalent circuit shown in FIG. 8. The power supply 105A applies a voltage V1 to the first-stage electrodes 102a, 102b. The power supply 105B applies a voltage V2 to the second-stage electrodes 103a, 103b. The electrodes 102a, 102b, 103a, 103b have electric resistances R1, R2, R3, R4, respectively, and the object 107 has an electric resistance Rb. There are electric resistances r1, r2, r3, r4 presented by the coating material 110 between the first-stage electrode 102a and the object 107, between the first-stage electrode 102b and the object 107, between the second-stage electrode 103a and the object 107, and between the second-stage electrode 103b and the object 107, respectively. There are also electric resistances r13, r23 presented by the coating material 110 between the second-stage electrode 103a, which is closer to the first-stage electrodes 102a, 102b, and the first-stage electrode 102a and between the second-stage electrode 103a and the first-stage electrode 102b, respectively.
When a switch 109 shown in FIG. 7 is turned on to connect the negative terminals of the power supplies 105A, 105B to each other, or the object 107 is electrically coupled to the second-stage bus bar 106b, a current loop is formed from the positive terminal of the power supply 105B successively through the resistances R3, r13, R1, R2, r2, Rb to the negative terminal of the power supply 105B, or from the positive terminal of the power supply 105B successively through the resistances R3, r23, R2, R1, r1, Rb to the negative terminal of the power supply 105B. Therefore, the first-stage electrode 102a or 102b is electrocoated with the coating material.